The present invention relates to a valve mechanism for low temperature applications, and more particularly to an improved valve mechanism suitable for use at cryogenic temperatures.
A spring type safety valve is conventionally used on pressure vessels such as boilers and compressed air tanks to prevent failure of the vessels due to excessively high pressure therein. However, no safety valves particularly designed for use at cryogenic temperatures, for example in a cryostat, are available. Consequently, conventional spring type safety valves are normally used on pressure vessels such as liquid helium vessels and others which are used under cryogenic temperatures.
However, use of conventional safety valves designed for normal temperatures at cryogenic temperatures has inevitably involved the following troubles. First, the seal members of the conventional valves deteriorate at cryogenic temperatures and soon malfunction. Second, the elasticity of a spring fluctuates at cryogenic temperatures, making it difficult to specify both set pressure and reset pressure correctly. There is also a need for a safety valve which can be selectively retained in an open state in addition to working as a normal safety valve.
A conventional cryostat for a super-conducting magnet has both a safety valve and an ordinary valve for opening and closing its outlet. Problems have been encountered concerning maintenance of such valves and concerning the heat loss through the valves. Particularly, in a cryostat for a magnetic floating train system, it is necessary to facilitate and expedite the maintenance of the valves to provide for the public convenience and safety.